It is well known that normal clothing fabric when used in environments such as a clean room, hospital operating room or other areas where humidity is reduced or controlled, have the disadvantage of becoming charged with static electricity as a result of friction. Due to the reduced humidity the static electricity is not readily dissipated into the air and in extreme circumstances could even result in an undesirable and dangerous spark. Likewise, when ordinary cloth is used as a filter medium, the friction caused by the air and particles passing through the cloth results in the buildup of static electricity.
In clean room environments where semiconductors are manufactured even the existence of an electrical charge of relatively low voltage on the garments of workers could cause damage to the semiconductors. Thus, it is desirable to have the rapid yet controlled electrostatic discharge to prevent the residual voltage from adversely affecting the manufacturing process.
There is a need for electrostatic dissipating fabrics which can be used to construct garments, filter media and drop cloths which have characteristics appropriate for each end use. For example, the fabric used for the production of garments must be comfortable against the skin, washable without loss of its static dissipating properties, must be economical, and should be relatively opaque. For filter media, the electrostatic dissipating fabric should have a uniform and controllable permeability to air or other gases passing through the filter and should have relatively low permeability to prevent particulate contamination from passing through the filter medium. For drop cloth applications, the electrostatic dissipating fabric should be reasonably economical and have a smooth enough surface to avoid snagging or damaging the object being covered.
In U.S. Pat. No. 2,845,962 an anti-static fabric is disclosed which is made from a fibrous material containing electrically conductive carbon black in combination with fibrous material free from the carbon black. Such fabric, however, does not have the desired conductivity unless a substantial amount of carbon black is used which would result in reduced mechanical strength of the fiber, often leading to the fabric breaking during the processing step. Further, the appearance and hand of the product produced from fiber containing a substantial amount of carbon black is generally unsatisfactory.
U.S. Pat. No. 3,288,175 teaches the incorporation of a small quantity of metallic fibers in the textile fiber materials to produce an anti-static fabric. However, such fabric is undesirable as it is difficult to process and has an unsatisfactory hand. Further, a fabric produced in accordance with such method is relatively expensive.
U.S. Pat. No. 3,586,597 teaches the use of a fiber which is coated with a resinous matrix of finely divided silver or carbon black. However, such coating is not always sufficient to provide the necessary anti-static properties and often results in loss of anti-static properties as a result of normal processing such as dying, cutting and so forth. In particular, when such fabric is washed further degradation of its anti-static properties occurs.
Likewise, the anti-static cloth described in U.S. Pat. No. 3,986,530, which is formed from an electrically conductive thread having electroless, metal plated, staple fibers and metallic filaments combined in a specific amount, suffers a degradation of anti-static performance from loss of the coating as the fabric is flexed or otherwise processed or washed.
Further, the fabric described in U.S. Pat. No. 4,232,082, although containing conductive and non-conductive yarns, is particularly constructed to mask the presence of electroconductive fibers. This is accomplished by covering the electroconductive fibers with non-conductive fibers. There are no raised grids on either side of the fabric and consequently it does not exhibit enhanced or directional dissipation of static electricity.